Method of making solid-state imaging device

ABSTRACT

A method for producing a solid-state imaging device including a photodetector including implanting two different dopant impurity ions, each producing the second conductivity type and having different diffusion coefficients in a first conductivity type semiconductor layer; thermally diffusing the implanted ions to produce a second conductivity type region including a relatively deep second conductivity type subregion and a relatively shallow second conductivity type region having a higher dopant impurity concentration than said relatively deep second conductivity type subregion; forming a charge transfer electrode on said semiconductor layer such that an edge of said electrode lies adjacent part of the junction between said semiconductor layer and said second conductivity type region; and implanting a dopant impurity producing the first conductivity type in said second relatively shallow second conductivity type subregion using said charge transfer electrode as a mask to produce a first conductivity type impurity diffusion region shallower than said relatively shallow second conductivity type subregion.

This application is a division of application Ser. No. 07/811,118, nowU.S. Pat. No. 5,191,399 filed Dec. 20, 1991.

FIELD OF THE INVENTION

The present invention relates to a solid-state imaging device and aproduction method therefor and, more particularly, to a construction anda production method of a photo detector as a component of thesolid-state imaging device.

BACKGROUND OF THE INVENTION

FIG. 3 is a diagram illustrating a cross section of a prior artsolid-state imaging device disclosed in the International ElectronDevices Meeting, 1982, p.324. In FIG. 3, reference numeral 1 designatesan n type Si substrate. A first p conductivity type region 2 is producedin the Si substrate 1. A first n conductivity type region 4 which is tobe a channel of a charge coupled device (hereinafter referred to asCCD), a second n conductivity type region 5 carrying out the photodetection, and a second p conductivity type region 3 serving as achannel stopper are produced in the first p conductivity type region 2.A third p conductivity type region 6 is produced in the second nconductivity type region 5 to suppress the generation of dark current atthe interface between the Si substrate 1 and an SiO₂ film 7. A chargetransfer electrode 8 of the CCD comprises a polysilicon film formed onthe surface of the element.

FIG. 4 is a diagram showing the impurity concentration distributiontaken along the line B--B' of FIG. 3. As shown in FIG. 4, impurityconcentration distributions are formed in the order of p, n, p, and ntype from the surface of the element.

A description is given of the operation.

When light is incident to the element from the above, photoelectrons aregenerated in the depletion layers adjacent to the pn junctions which areproduced between the second n conductivity type region 5 and the firstand third p conductivity type regions 2 and 6 and the photoelectrons arestored in the second n conductivity type region 5. When a high levelvoltage is applied to the polysilicon film 8 serving as a chargetransfer electrode after a predetermined storage time, thephotoelectrons are transferred to the channel 4 of the CCD and furtheroutput to the outside.

FIG. 7 is a diagram showing an electric potential distribution takenalong the line B--B' of FIG. 3. The function of the third p conductivitytype region 6 is described in detail using this figure.

The second n conductivity type region 5 is intended to be completelydepleted at the time when photoelectrons are transferred to the channel4 of the CCD. Otherwise, remaining electrons result an afterimagethrough the thermal diffusion process. However, when there is no third pconductivity type region 6, complete depletion of the second nconductivity type region 5 means that the depletion extends to theinterface with the SiO₂ film 7. There is a defect 16 which produces anenergy level in the energy band gap of the silicon at the SiO₂ interfaceand it functions as a generating center of dark current charge carriers.Accordingly, the third p conductivity type region 6 is provided toproduce accumulated holes, thereby suppressing the generation of thedark current caused by the defect at the interface.

A description is given of the production method.

FIGS. 5(a) to 5(c) are cross-sectional diagrams illustrating aproduction process of the solid-state imaging device having theabove-described construction.

Firstly, boron is implanted into the n type Si substrate 1 to producethe first p conductivity type region 2. Phosphorus and boron areselectively implanted through a resist mask and then they are diffusedrespectively through annealing process to produce the second pconductivity type region 3, the first n conductivity type region 4, andthe second n conductivity type region 5 (FIG. 5(a)).

Next, the SiO₂ film 7 is formed on the surface of the Si substrate 1 bya thermal oxidation method. The polysilicon film 8 is deposited usingthe chemical vapor deposition (CVD) method and thereafter thepolysilicon film 8 is patterned into a desired configuration (FIG.5(b)).

Next, boron 10 is implanted into the entire surface of the element at anenergy so that the boron cannot transit the polysilicon film 8 but cantransit the SiO₂ film 7, and then the boron 10 is activated by annealingto produce the third p conductivity type region 6.

In the above-described processes, the boron 10 for producing the third pconductivity type region 6 is implanted after patterning the polysiliconfilm 8 which is to be a charge transfer electrode of the CCD, andthereby the spreading of boron having a large diffusion coefficient issuppressed. If boron spreads to such an extent that the third pconductivity type region 6 extends to the deep portion of the nconductivity type region 5, the effective impurity concentration of then conductivity type region 5 which stores photoelectrons will be reducedand the storage capacitance is lowered.

The prior art solid-state imaging device constituted as described abovehas typically two problems.

First, although the third p conductivity type region 6 is produced at alater process so that it spreads as little as possible, it is notpossible to avoid annealing. For example, a reflow treatment forproducing a flattening film on the polysilicon film 8 is carried out athigh temperature or about 900° C. Therefore, the boron ions implanted inthe process step of FIG. 5(c) are further diffused during the laterprocess, thereby reducing the effective impurity concentration of thesecond n conductivity type region 5 which determines the total storagecharge amount of the photodiode. In addition, since the pn junction isdeep relative to the surface, the sensitivity to blue light which isabsorbed near the surface region is lowered.

Second, the electron potential φ_(PD) and the maximum storage chargeamount Q_(PD) when the second n conductivity type region 5 is depleted,shown in FIG. 7 vary as shown in FIG. 8 in response to the impurityconcentration of the n conductivity type region 5. While the maximumstorage charge amount Q_(PD) needs to be more than a predetermined value(shown by an upward directed arrow in FIG. 8), the electron potentialφ_(PD) needs to be less than a potential value determined by the voltageapplied to the charge transfer electrode of the CCD (shown by a downwarddirected arrow in FIG. 8). The relation between the maximum storagecharge amount Q_(PD) and the electron potential φ_(PD) can berepresented by the formula Q_(PD) =C_(PD) ×φ_(PD), using the capacitanceC_(PD) of the depletion layer adjacent the pn junction. Accordingly, inorder to satisfy the above-described two conditions, the maximum storagecharge amount Q_(PD) can be increased by increasing the capacitanceC_(PD) of the depletion layer, that is, by narrowing the depletion layerat both sides of the second n conductivity type region 5 even keepingthe same electron potential φ_(PD). However, incident light is convertedto charge carriers in the depletion layer and the number of generatedelectrons as a function of the incident light amount depends on thewidth of the depletion layer. Therefore, when the depletion layerextending at both sides of the second n conductivity type region 5 isnarrowed to increase the capacitance C_(PD) of the depletion layer, thesensitivity of the element is reduced.

As described above, in the prior art solid-state imaging device, it isdifficult to form a shallow and high-impurity concentration pconductivity type region 6 that reduces the generation of dark currentat the surface of the photodiode. It is also difficult to satisfy thedesired conditions for the electron potential and for the maximumstorage charge amount required to maintain good sensitivity.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a solid-stateimaging device that has a good blue-color sensitivity and that canincrease the maximum storage charge amount in response to each potentialvalue without loss in the sensitivity.

It is another object of the present invention to provide a productionmethod therefor.

Other objects and advantages of the present invention will becomeapparent from the detailed description given hereinafter; it should beunderstood, however, that the detailed description and specificembodiment are given by way of illustration only, since various changesand modifications within the spirit and the scope of the invention willbecome apparent to those skilled in the art from this detaileddescription.

According to a first aspect of the present invention, a photo detectorof a solid-state imaging device comprises a first conductivity typesemiconductor layer in which a second conductivity type semiconductorlayer is formed, and a first conductivity type impurity diffusion layerformed in the surface region of the second conductivity typesemiconductor layer. The second conductivity type semiconductor layercomprises at least two second conductivity type impurity diffusionlayers having different impurity concentrations, and the secondconductivity type impurity diffusion layer which is in contact with thefirst conductivity type impurity diffusion layer has an impurityconcentration higher than that of the other second conductivity typeimpurity diffusion layers. Therefore, the junction capacitance betweenthe second conductivity type semiconductor layer, and the firstconductivity type diffusion layer at the periphery thereof and the firstconductivity type semiconductor layer are increased, so that the maximumstorage charge amount when the first conductivity type impuritydiffusion layer is depleted is increased at the same potential. Inaddition, the second conductivity type semiconductor layer has a lowimpurity concentration and a deep pn junction, thereby preventing thedeterioration of the sensitivity due to expansion of the depletion layerin the depth direction.

According to a second aspect of the present invention, a method forproducing a solid-state imaging device includes selectively forming asecond conductivity type impurity diffusion region in a firstconductivity type semiconductor layer and forming a charge transferelectrode such that an end portion thereof coincides with that of thesecond conductivity type impurity diffusion region, implanting a secondconductivity type impurity into the surface region of the secondconductivity type impurity diffusion region using the charge transferelectrode as a mask to produce a second conductivity type high impurityconcentration region that has an impurity concentration higher than thatof the second conductivity type impurity diffusion region and a shallowjunction, and implanting a first conductivity type impurity into thesurface of the second conductivity type high impurity concentrationregion using the charge transfer electrode as a mask to produce a firstconductivity type impurity diffusion layer that has a shallower junctionthan the second conductivity type high impurity concentration region.Therefore, the effective profile of the first conductivity type impuritycan avoid expansion toward the lower side owing to the existence of thesecond conductivity type high impurity concentration region at theannealing process. Accordingly, production of the pn junction at a deepposition is prevented. Further, the maximum storage charge amount is notreduced and the blue-color sensitivity is not deteriorated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional side view of a solid-state imaging device inaccordance with a first embodiment of the present invention;

FIG. 2 is a diagram illustrating an impurity concentration distributiontaken along line A--A' of FIG. 1;

FIG. 3 is a sectional side view of a prior art solid-state imagingdevice;

FIG. 4 is a diagram illustrating an impurity concentration distributiontaken along line B--B' of FIG. 3;

FIGS. 5(a) to 5(c) are cross-sectional views illustrating a prior artmethod for producing a solid-state imaging device;

FIGS. 6(a) and 6(b) are cross-sectional views illustrating a method forproducing a solid-state imaging device of FIG. 1;

FIG. 7 is a energy band diagram of the photo detector portion in thesolid-state imaging device taken along line B--B' of FIG. 3;

FIG. 8 is a diagram showing the dependency of the potential and on theimpurity concentration in the second n conductivity type region;

FIG. 9 is a diagram illustrating an equivalent circuit of the regionaround the photo detector of the solid-state imaging device;

FIGS. 10(a) and 10(b) are cross-sectional views illustrating a methodfor producing a solid-state imaging device in accordance with a thirdembodiment of the present invention;

FIGS. 11(a) and 11(b) are impurity concentration distribution diagramsfor explaining a method for producing a solid-state imaging device inaccordance with a fourth embodiment of the present invention; and

FIG. 12 is a cross-sectional view of a solid-state imaging device inaccordance with second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will be described in detail withreference to the drawings.

FIG. 1 is a cross section of a solid-state imaging device in accordancewith a first embodiment of the present invention. In FIG. 1, the samereference numerals as those of FIG. 3 designate the same orcorresponding portions. A second n conductivity type region 5' has thesame or deeper junction and a lower impurity concentration compared withthe conventional one because the ion implantation amount is decreasedand the impurity diffusion by annealing is increased. A third nconductivity type region 9 having a higher impurity concentration thanthat of the second n conductivity type region 5' (a second conductivitytype high impurity concentration region) is provided between the third pconductivity type region 6 and the second n conductivity type region 5'.FIG. 2 is a diagram showing the impurity concentration distributiontaken along line A--A' of FIG. 1. FIG. 9 is a diagram illustrating anequivalent circuit around the photo detector.

An operation and effects will be described.

The third n conductivity type region 9 has a higher impurityconcentration than in the second n conductivity type region 5' asdescribed above. Accordingly, in FIG. 9, the depletion layer is notexpanded, resulting in a large junction capacitance C₁ with the third pconductivity type region 6. This is because the junction capacitance C₁and the depletion layer W are related by:

    C.sub.1 ε.sub.O ε.sub.S /W,

the width of depletion layer W is represented by: ##EQU1## and, thewidth of the depletion layer W is decreased and the junction capacitanceC₁ is increased as the impurity concentration N_(D) of the third nconductivity type region 9 is increased. Herein, (ε_(S) ε_(O))represents a dielectric constant of silicon, q represents the elementarycharge quantity of an impurity ion, φ_(PD) represents the potential whenthe second and the third n conductivity type regions 5' and 9 aredepleted, and N_(A) represents an impurity concentration of the third pconductivity type region 6.

Since the second n conductivity type region 5' is formed having a lowerimpurity concentration and a deeper junction than in the conventionaldetector shown in FIG. 2, depletion layers which are extended at bothsides of the junction between the second n conductivity type region andthe first p conductivity type region are wider than the conventionaldetector. Therefore, the photoelectric conversion capability for theincident light is increased, resulting in increased the sensitivity.That is, in the equivalent circuit of FIG. 9, while the junction regionbetween the second n conductivity type region 5' and the first pconductivity type region 2 has a wide depletion layer and a smalljunction capacitance C₂, the junction capacitance C₁ is established at alarge value, resulting in the total junction capacitance C_(PD) (=C₁+C₂) having rather a large value. Accordingly, since the maximum storagecharge amount Q_(PD) of the second n conductivity type region 5' isrepresented by Q_(PD) =C_(PD) φ_(PD), the maximum storage charge amountQ_(PD) for a predetermined potential φ_(PD) of the second n conductivitytype region can be increased without reducing the sensitivity.

Next, a description will be given of a method for producing thesolid-state imaging device of this embodiment with reference to FIGS.5(a)-5(c) and 6(a)-6(b).

By the same steps as those of the prior art shown in FIGS. 5(a) and5(b), the second p conductivity type region 3, the first n conductivitytype region 4, and the second n conductivity type region 5' are formedin the first p conductivity type region 2 formed in the substrate 1.Then, the second n conductivity type region 5' is formed having the sameor deeper junction and a lower impurity concentration compared to theprior art by reducing the ion implantation amount of n type impurity andperforming the impurity diffusion by annealing.

Next, as shown in FIG. 6(a), a photoresist film 12 is deposited andpatterned by a photolithography process and then phosphorus 11 isimplanted as an n type impurity from an aperture 13 of the photoresistfilm 12. At this time, the aperture 13 is produced on a region includingan edge portion of the polysilicon film 8 serving as a charge transferelectrode of the CCD as shown in FIG. 6(a), whereby the phosphorusimplantation region is self-aligned with the polysilicon film 8.

Next, as shown in FIG. 6(b), boron 10 is implanted as a p type impurityinto the entire surface after removing the photoresist film 12.Thereafter, the phosphorus and boron ions ion are activated by annealingto produce conductive regions having predetermined depths such that thethird n conductivity type region 9 is shallower than the second nconductivity type region 5' and deeper than the third p conductivitytype region 6 shown in FIG. 3. To realize such a structure, it isnecessary that the implantation energy of boron in the step of FIG. 6(b)is as small as possible and that the implantation energy of phosphorusin the step of FIG. 6(a) is larger than that of boron but smaller thanthe energy which is required to pass through the polysilicon film 8.

The above-described structure suppresses the blooming phenomenongenerated in a case where a strong light is incident. The first pconductivity type region 2 under the photo detector can be alwayscompletely depleted and can draw excessive electrons toward the nconductivity type silicon substrate 1. Therefore, the depth of thesecond n conductivity type region 5' and the depth and impurityconcentration of the first p conductivity type region 2 is limited to arange so that electrons can be drawn toward the substrate. However, thethird n conductivity type region 9 may be shallow because the depththereof has no relation to the electron drawing operation. For example,when the depth of the first p conductivity type region 2 is 3 to 4microns, the depth of the second n conductivity type region 5' needs tobe 1 to 2 microns while the depth of the n conductivity type region 9may be less than 0.5 micron.

In the above-described embodiment, the photo detector of the solid-stateimaging device comprises the p conductivity type region comprising thefirst and third p conductivity type regions 2 and 6 and the nconductivity type region comprising the second and third n conductivitytype regions 5' and 9 which are disposed between the two p conductivitytype regions, and the impurity concentration of the third n conductivitytype region 9 is made higher than that of the second n conductivity typeregion 5'. Therefore, the junction capacitance between the secondconductivity type region and the first and third p conductivity typeregions 2 and 6 at the periphery thereof is large, so that the maximumstorage charge amount Q_(PD) is increased when the layers are depletedby the same potential. In addition, the second conductivity typesemiconductor layer has a low concentration and a deep junction, therebyexpanding the depletion layer in the depth direction, resulting in nodeterioration in sensitivity.

Furthermore, the second conductivity type region comprises the second nconductivity type region 5' and the third n conductivity type region 9having a higher concentration than that of the second n conductivitytype region 5'. Therefore, when the p conductivity type impurity such asboron is implanted into the third n conductivity type region andannealed, or when an annealing is carried out at the reflow process forproducing a flattening film thereafter, expansion of the effectiveprofile of the third p conductivity type region 9 toward the layer 2 isavoided owing to the existence of the second conductivity type highimpurity concentration region. Therefore, it is possible to prevent theformation of pn junctions at a deep position. Further, the maximumstorage charge amount is not reduced and the bluecolor sensitivity isnot deteriorated.

In the above-described embodiment shown in FIG. 1, respective edgeportions of the second and third n conductivity type regions 5' and 9and of the third p conductivity type region 6 coincide with one anotherat left end. As shown in a second embodiment of FIG. 12, however, thoseedges may be shifted a little toward the electrode 8 successively fromthe upper layer to the lower layer on a condition that the third pconductivity type region 6 is in touch with the second p conductivitytype region 3 which serves as a channel stopper.

While in the above-described first embodiment boron implantation iscarried out after phosphorus implantation, the boron ion implantationmay be carried out before phosphorus is implanted.

Furthermore, the boron ion implantation may be carried out after thatphosphorus ion implantation and diffusion by annealing are carried out.In this case, there is a possibility that the second conductivity typeregion and the polysilicon film 8 will not be self-aligned. There alsomay occur difficulty in reading out the signal charges due to adepression of the potential which is generated at overlapping portionsof the two regions.

While in the above described embodiment phosphorus is used as the n typeimpurity, arsenic may be used therefor.

In a case where the second p conductivity type region 3 serving as anelement separating region has a sufficiently high do pant concentrationcompared with the third n conductivity type region 9, the ionimplantation of phosphorus shown in FIG. 6(a) may be performed to theentire surface without using the photoresist film 12.

FIGS. 10(a) and 10(b) are cross sectional views showing a method forproducing a solid-state imaging device in accordance with a thirdembodiment of the present invention. In the figures, the same referencenumerals as those of FIG. 1 designate the same or correspondingportions.

A photoresist (a second photoresist film 17) used for patterning thepolysilicon film 8 at the steps shown in FIG. 5(b) is not removed asshown in FIG. 10(a), and then at the step corresponding to that of FIG.6(a), a photoresist film 12 is formed and patterned with the secondphotoresist film 17 remaining such that the polysilicon 8 and thephotoresist film 17 deposited thereon exists within the aperture 13 asshown in FIG. 10(b). When phosphorus 11 is implanted in this state toform the n conductivity type region 9, the upper limit of the selectionrange of the phosphorus ion implantation energy can be made higherbecause the upper limit of the ion implantation energy is decideddepending on the thickness of the photoresist film 17. While the upperlimit of the phosphorus ion implantation energy is about 170 KeV for apolysilicon film 5000 angstroms thick in the first embodiment, it can beup to about 650 KeV employing the photoresist film 17 2 microns thick inthe second embodiment. Thus, the degree of freedom in producing thethird n conductivity type region 9 is largely increased.

Next, a fourth embodiment of the present invention is described.

In the above-described first and third embodiments, a first ionimplantation of an n conductivity type impurity is carried out followedby an annealing process, and then a second ion implantation process ofthe same n conductivity type impurity is carried out to produce the nconductivity type regions having two-stage impurity concentrationdistribution (5' and 9). However, two kinds of n type impurities havingdifferent diffusion coefficients may be used at the step of forming thesecond n conductivity type region 5'. That is, as shown in FIG. 11(a),phosphorus and arsenic are implanted successively without annealing, toform phosphorus-arsenic implanted layer 18. Thereafter, annealing iscarried out to diffuse these impurities. Since the diffusion coefficientof phosphorus is smaller than that of arsenic by more than one order ofmagnitude, two stages of the concentration distribution comprising anarsenic diffusion layer 19 and a phosphorus diffusion layer 20 can beobtained as shown in FIG. 11(b).

As is evident from the foregoing description, according to theinvention, a photo detector of a solid-state imaging device comprises anupper first conductivity type impurity diffusion layer, a lower firstconductivity type semiconductor layer, and a second conductivity typesemiconductor layer disposed therebetween. The second conductivity typesemiconductor layer comprises two second conductivity type impuritydiffusion layers having different impurity concentration distributions,and the two second conductivity type impurity diffusion layer which isin contact with the first conductivity type impurity diffusion layer hasa higher concentration than that of the other second conductivity typeimpurity diffusion layer, thereby producing a solid-state imaging devicehaving a high sensitivity and a large maximum storage charge amount.

In addition, according to the present invention, a method for producinga solid-state imaging device includes forming a second conductivity typehigh impurity concentration layer having an impurity concentrationhigher than that of a second conductivity type semiconductor layer inthe second conductivity type semiconductor layer constituting a photodetector, and of implanting a first conductivity type impurity into thesurface of the second conductivity type high impurity concentrationregion and annealing to form a first conductivity type impuritydiffusion layer. Therefore, the first conductivity type impurityprevents expansion toward the second conductivity type semiconductorlayer side at the annealing process. Therefore, the first conductivitytype impurity diffusion layer can be shallow and having a high impurityconcentration, whereby it is possible to prevent the complete depletionof the first conductivity type semiconductor layer deposited between thesubstrate and the second conductivity type semiconductor layer and thegeneration of dark current. Further, the blue light sensitivity is notreduced.

What is claimed is:
 1. A method for producing a solid-state imagingdevice including a photodetector comprising:selectively forming a secondconductivity type region in a first conductivity type semiconductorlayer; forming a charge transfer electrode on said semiconductor layersuch that an edge of said electrode lies adjacent part of the junctionbetween said semiconductor layer and said second conductivity typeregion; implanting a dopant impurity producing the second conductivitytype in said second conductivity type region using said charge transferelectrode as a mask to produce a second conductivity type subregionwithin said second conductivity type region that has an impurityconcentration higher than the remainder of said second conductivity typeregion; and implanting a dopant impurity producing the firstconductivity type in said subregion using said charge transfer electrodeas a mask to produce a first conductivity type region shallower thansaid second conductivity type subregion.
 2. A method for producing asolid-state imaging device including a photodetectorcomprising:implanting two different dopant impurity ions, each producingthe second conductivity type and having different diffusion coefficientsin a first conductivity type semiconductor layer; thermally diffusingthe implanted ions to produce a second conductivity type regionincluding a relatively deep second conductivity type subregion and arelatively shallow second conductivity type region having a higherdopant impurity concentration than said relatively deep secondconductivity type subregion; forming a charge transfer electrode on saidsemiconductor layer such that an edge of said electrode lies adjacentpart of the junction between said semiconductor layer and said secondconductivity type region; and implanting a dopant impurity producing thefirst conductivity type in said second relatively shallow secondconductivity type subregion using said charge transfer electrode as amask to produce a first conductivity type impurity diffusion regionshallower than said relatively shallow second conductivity typesubregion.
 3. The method for producing a solid-state imaging device ofclaim 1 including implanting a dopant impurity producing the secondconductivity type in said layer spaced from said second conductivitytype region as a channel layer for transferring charge carriers.
 4. Themethod for producing a solid-state imaging device of claim 2 includingimplanting a dopant impurity producing the second conductivity type insaid layer spaced from said second conductivity type region as a channellayer for transferring charge carriers.